Mistake in ComputedProperty example

Under https://api.emberjs.com/ember/release/classes/ComputedProperty the fullNameMacro decorator needs to be invoked in the Person class:

```
class Person {
constructor(firstName, lastName) {
set(this, 'firstName', firstName);
set(this, 'lastName', lastName);
}

@fullNameMacro('firstName', 'lastName') fullName;
});
```
For the example below,
```
import { computed, set } from '@ember/object';

function fullNameMacro(firstNameKey, lastNameKey) {
return computed(firstNameKey, lastNameKey, {
get() {
return `${this[firstNameKey]} ${this[lastNameKey]}`;
}

set(key, value) {
let [firstName, lastName] = value.split(' ');

set(this, firstNameKey, firstName);
set(this, lastNameKey, lastName);

return value;
}
});
}

class Person {
constructor(firstName, lastName) {
set(this, 'firstName', firstName);
set(this, 'lastName', lastName);
}

@fullNameMacro fullName;
});

let person = new Person();

set(person, 'fullName', 'Peter Wagenet');
person.firstName; // 'Peter'
person.lastName; // 'Wagenet'
```